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layout_of: T: Thin implies sizeof(&T) == sizeof(usize)

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This commit is contained in:
Michael Goulet 2022-11-12 23:54:39 +00:00
parent c7572670a1
commit b25da9ce6d
2 changed files with 39 additions and 8 deletions
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36
compiler/rustc_ty_utils/src/layout.rs
View File

@ -155,17 +155,37 @@ fn layout_of_uncached<'tcx>(
}
let unsized_part = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
let metadata = match unsized_part.kind() {
ty::Foreign(..) => {
let metadata = if let Some(metadata_def_id) = tcx.lang_items().metadata_type() {
let metadata_ty = tcx.normalize_erasing_regions(
param_env,
tcx.mk_projection(metadata_def_id, [pointee]),
);
let metadata_layout = cx.layout_of(metadata_ty)?;
// If the metadata is a 1-zst, then the pointer is thin.
if metadata_layout.is_zst() && metadata_layout.align.abi.bytes() == 1 {
return Ok(tcx.intern_layout(LayoutS::scalar(cx, data_ptr)));
}
ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
ty::Dynamic(..) => {
let mut vtable = scalar_unit(Pointer);
vtable.valid_range_mut().start = 1;
vtable
let Abi::Scalar(metadata) = metadata_layout.abi else {
return Err(LayoutError::Unknown(unsized_part));
};
metadata
} else {
match unsized_part.kind() {
ty::Foreign(..) => {
return Ok(tcx.intern_layout(LayoutS::scalar(cx, data_ptr)));
}
ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
ty::Dynamic(..) => {
let mut vtable = scalar_unit(Pointer);
vtable.valid_range_mut().start = 1;
vtable
}
_ => {
return Err(LayoutError::Unknown(unsized_part));
}
}
_ => return Err(LayoutError::Unknown(unsized_part)),
};
// Effectively a (ptr, meta) tuple.

11
src/test/ui/layout/thin-meta-implies-thin-ptr.rs Normal file
View File

@ -0,0 +1,11 @@
// check-pass
#![feature(ptr_metadata)]
use std::ptr::Thin;
fn main() {}
fn foo<T: ?Sized + Thin>(t: *const T) -> *const () {
unsafe { std::mem::transmute(t) }
}